1. Field of the Invention
This invention relates to an ophthalmic apparatus capable of measuring the shape of the cornea of an eye to be examined.
2. Related Background Art
In an apparatus for measuring the shape of the cornea of an eye to be examined, as disclosed in U.S. Pat. No. 4,660,946, a ring-shape index mark or a plurality of calescence point index marks disposed on the circumference of the same circle are projected onto the cornea of an eye to be examined, and from the shapes and positions of the reflected images thereof, an ellipse passing through those reflected images is specified, whereby the shape of the cornea is calculated and measured.
In the above-described example of the prior art, however, even in the case of a cornea of so-called bad astigmatism the shape differs from the normal shape of which calculation and measurement is effected so that an ellipse may be forcibly formed, and this has led to the problem that in spite of being a cornea of bad astigmatism, such a cornea is mistaken for a cornea of normal shape. This is attributable to the fact that in the above-described example of the prior art, the center of the ellipse is indirectly found by calculation, and this means that there is the above-noted problem with regard to bad astigmatism.
It is proposed in Japanese Laid-Open Patent Application No. 59-118131 by the assignee of the present invention to provide a plurality of concentric ring-shape index marks so that the measurement of the shape of a cornea may be possible, even if the cornea has bad astigmatism, indirectly to find the center of an ellipse by calculation on the basis of one of the ring-shaped index mark images, and to find the shape of the cornea by the center of the ellipse and the other ring-shaped index mark images. However, this also fins indirectly the center of the ellipse by calculation as described above and therefore suffers from the problem that if the cornea has bad astigmatism, the original center of the ellipse cannot be specified.